Wireless technology has become an integral part of society with widespread use of such devices as the pager and cellular phone, as well as networking technology such as wireless routers. With the explosion in use of wireless technology, there are many instances where a nearby wireless transmitter may interfere with an adjacent receiver. Under these circumstances, it is possible to remove the offending transmitter signal at the receiver's frequency by placing a band-reject filter at the output of the transmitter and tuning the band-reject filter to the frequency of the adjacent receiver.
Band reject filters find utility in canceling interference in a number of wireless technologies such as cellular phone, wireless routers, hand-held radios, satellite communications, and any other situation where there may be a number of wireless devices in close proximity. Conventional, non-absorptive filters reflect power at frequencies in the reject band, which can create undesirable electromagnetic interference, as well as, damage electronic components if the reflected power is too large. As the radio frequency (RF) power level of transmitters increase, it becomes a problem to use conventional band-reject filters.
An example of a commercially available conventional band-reject filter is Model U2916 band-reject filter offered by Delta Microwave Inc. at 300 Del Norte Boulevard, Oxnard, Calif. 93030. As illustrated in FIG. 1, the high return losses 1 of such conventional filters in the reject band are the result of the power at frequencies in the reject band being reflected back to the transmitter. The insertion losses 2 are also shown in FIG. 1. At low RF power levels, the reflected power can interact with the transmitted power to create interference signals known as intermodulation distortion products. At high RF power levels, the reflected power can physically damage the transmitter.
While it may be desirable to provide a band-reject filter with an absorptive response, it is also desirable to have a pass-band over a very wide frequency range because RF systems can operate over a maximum-to-minimum frequency range ratio exceeding 100:1. For example, modern digital radios, each operating over several octaves of frequencies, can be multiplexed together to cover very wide frequency ranges. There have been published methods for achieving band-reject filters or wide bandwidth all-pass networks, but none have reported the ability to create an absorptive notch filter with a pass-band that operates over a very wide (100:1 or more) frequency range. Therefore, there is a need for an absorptive band-reject filter that also operates with a pass-band over a very wide (100:1 or more) frequency range bandwidth.
In other prior art, U.S. Pat. No. 3,748,601, entitled “Coupling Networks Having Broader Bandwidth than Included Phase Shifters”, issued to Harold Seidel on Jul. 24, 1973, describes a technique for extending the bandwidth of a quadrature hybrid coupler using a phase shifter. However, this disclosure does not provide the advantages of a wide pass-band, absorptive band-reject filter that reduces the insertion loss of the quadrature hybrid coupler and the overall topology.
U.S. Published Patent Application 2009/0289744, entitled “Electronically Tunable, Absorptive, Low-Loss Notch Filter”, filed in the name of Kevin Miyashiro, and owned in common with the present patent application, describes a technique for creating an absorptive band-reject filter, but its bandwidth is limited by the quadrature hybrids used.
FIG. 12 illustrates pass-bands of three different all-pass networks. The dotted line plot 33 indicates a wide frequency pass band range for an all-pass network. FIG. 13 illustrates the components in a conventional all-pass network having two cascaded quadrature hybrid couplers 3 and 7 in parallel coupled in one path by a 180-degree phase shifter 4, similar to that described in U.S. Pat. No. 3,748,601. However, the all-pass network of the prior art cannot perform the band-reject function to prevent interference from a transmitter on an adjacent receiver while maintaining the wide pass-band. When quadrature hybrid couplers are used in a shunt configuration as described in U.S. Published Patent Application 2009/0289744, an absorptive response in the reject band is achieved, but the pass-band is limited to frequency ranges of 20:1 because the response is limited by the bandwidth of the quadrature hybrid couplers. The solid line 34 in FIG. 12 indicates the pass band using this technique, but it does not extend to low frequencies. The wide pass band also cannot be achieved by cascading two quadrature hybrid couplers without a phase shifter in one of the parallel paths. The dashed line 35 in FIG. 12 indicates that the frequency range with this technique is also limited and does not extend to low frequencies.
U.S. Pat. No. 7,323,955, entitled “Narrow-band Absorptive Bandstop Filter with Multiple Signal Paths,” issued to Douglas R. Jachowski on Jan. 29, 2008, describes a technique for achieving absorptive band-reject filters using a quarter-wave transmission line, but whose band-pass bandwidth is limited by the narrow bandwidth of the quarter-wave transmission line.